A conventional reduced iron production process imposes restrictions, such as a need to use costly natural gas as a reducing agent, and a plant location generally limited to localities of natural gas. For this reason, great interest has recently been shown in a reduced ion production process using, as the reducing agent, coal which is relatively low in cost and capable of easing geographic restrictions on selection of a plant location.
As one of the techniques for producing reduced iron using coal, the applicant of this application proposed a method which comprises agglomerating a powdery mixture of coal and an iron oxide-containing material, such as iron ore or steel mill dust, to produce a carbonaceous material-containing metal oxide, and charging the carbonaceous material-containing metal oxide into a rotary hearth furnace to produce reduced iron through heating and reduction (see, for example, Patent Document 1).
Means for the agglomerating of a powdery mixture of coal and an iron oxide-containing material includes spherical pelletization based on tumbling granulation using a pelletizer, cylindrical pelletization based on mechanical extrusion, and briquetting based on pressing using a briquette roll.
However, means for the pelletization based on tumbling granulation has problems, in a process of subjecting a material containing a large amount of fine particles, such as steel mill dust, to tumbling granulation: the problems are 1) a granulation rate becomes lower to cause deterioration in productivity; 2) a large amount of water is required for the granulation due to a great specific surface area of the material, which increases an amount of heat required for drying (i.e., removing the water) in a subsequent process to thereby increase energy consumption; and 3) a variation of a particle size of the material makes the granulation unstable, which is likely to cause a variation in production output (see, for example, Patent Document 2).
The pelletization based on extrusion involves an use of a fluidized material, thus generally requiring an greater amount of water to be contained in the material than that in the tumbling granulation. This further increases a consumption of energy for drying in a subsequent process.
As means for the briquetting using a briquette roll, a double roll-type briquette machine as disclosed, for example, in Patent Document 3 (in the Patent Document 3, a mixer and a pressure forming machine), may be employed. This double roll-type briquette machine comprises a pressure roll adapted to be drivenly rotated by a motor, and a hopper adapted to supply a material to the pressure roll from thereabove, wherein a screw feeder for extruding the material is provided within the hopper.
The use of such a double roll-type briquette machine allows a liquid binder such as molasses or lignin to be used, thus enabling a dry material to be directly agglomerated without addition of water thereto. This makes it possible to drastically reduce the consumption of energy required for drying in a subsequent process.
However, the inventors have been found the following problems (1) to (3) in a process of producing briquettes using steel mill dust, such as electric arc furnace dust, containing a large amount of oil, by means of the double roll-type briquette machine.
(1) There is a need of a forcible feed of a fine-particle material such as electric arc furnace dust to a pressure roll by a transfer device such as a screw feeder, because the dust is hard to be reliably supplied into pockets of the pressure roll only by gravity; however, even the forcible feed will become difficult when the fine-particle material contains a large amount of oil, because it makes the material fairly slippy, thereby deviating an extrusion force of the transfer device (for example, in a screw feeder, deviating an extrusion force of the screw feeder outward in a radial direction thereof). This prevents improvement in strength of briquettes.
(2) In order to overcome the above problem (1) about a reduction in briquette strength, a rotation speed of the pressure roll must be extremely lowered so that the material can be reliably fed into the pockets of the pressure roll. This, however, causes significant reduction in briquette production capacity.
(3) If an amount of liquid binder is increased to simultaneously overcome the two problems (1) and (2) about a reduction in briquette strength and a degradation in briquette production capacity, the fine-particle material will stick onto the presser roll in the pockets thereof to thereby render the production of the briquettes difficult.
[Patent Document 1] JP 2004-269978A
[Patent Document 2] JP 2001-348625A
[Patent Document 3] JP 09-192896A